The present invention relates generally to the field of marine geophysical surveying. More particularly, in one or more embodiments, this invention relates to methods and systems for performing marine geophysical surveys that utilize a streamer having variable stiffness.
Techniques for marine surveying include marine geophysical surveying, such as seismic surveying and electromagnetic surveying, in which geophysical data may be collected from below the Earth's surface. Geophysical surveying has applications in mineral and energy exploration and production to help identify locations of hydrocarbon-bearing formations. Certain types of marine geophysical surveying, such as seismic or electromagnetic surveying, may include towing an energy source at a selected depth—typically above the seafloor—in a body of water. One or more geophysical sensor streamers also may be towed in the water at selected depths by the same or a different vessel. The streamers are typically cables that include a plurality of geophysical sensors disposed thereon at spaced apart locations along the length of the cable. The geophysical sensors may be configured to generate a signal that is related to a parameter being measured by the sensor. At selected times, the energy source may be actuated to generate, for example, seismic or electromagnetic (“EM”) energy that travels downwardly into the subsurface rock. Energy that interacts with interfaces, generally at the boundaries between layers of rock formations, may be returned toward the surface and detected by the geophysical sensors on the streamers. The detected energy may be used to infer certain properties of the subsurface rock, such as structure, mineral composition and fluid content, thereby providing information useful in the recovery of hydrocarbons.
In geophysical surveying, the streamer is typically a cable that is stored on a drum on the towing vessel. The streamers are typically made of multiple components, such as electrical conductors, fiber optics, and stress-supporting members, all bundled together and covered with a protective outer skin. The streamer may be up to several kilometers in length. In general, the streamer has little stiffness in directions other than inline, so it can move easily both laterally and in torsion/rotation when deployed in the water. When sensors such as velocity, position, and acceleration sensors are incorporated into the streamer, the movements are picked up directly by the sensors. Unlike hydrophones which only pick up the movements indirectly because of improvements over the years, these other sensors may have a high level of noise which is not interesting for the marine survey. For example, the noise may be measurements of local conditions in the surrounding water rather than reflections from the Earth below.
Under a load of pressure on only a small portion of the outside, a streamer will bend, held back only by the tension, bending and torsional stiffness of the streamer, and the mass of the cable content, depending on the direction, distribution, and size of the pressure. Low bending and torsional stiffness for the streamer should result in little added mass, but cause large local movement. As result, the streamer may have large local sensor recordings (i.e., measurements of local conditions in the surrounding water) and also large waves of motion traveling through the cable. Low stiffness may also result in large sagging of the cable between steering devices with wings commonly used to control lateral or vertical position of the streamer. Because of the large sagging, there may be large angles between the cable and fluid flow, further increasing turbulence and noise generation from hydrodynamic flow.
Accordingly, there is a need for improved methods and systems for deploying streamers in seismic surveys having increased stiffness both laterally and in torsion to reduce noise sources in marine seismic surveys.